Particulate filler material is typically incorporated into a matrix to enhance certain properties of the composite such as strength, color, bulk, etc. The matrix is typically a thermoplastic or thermoset polymeric resin. The particulate filler is usually treated with a silane to modify the surface properties of the filler and/or couple the filler to the matrix material. The silane treatment can lead to improvements in filler dispersion, chemical resistance, abrasion resistance, high temperature deformation, wet and dry electrical properties, scratch resistance, tensile strength, flexural strength, and other mechanical properties.
Silanes are commonly employed as treatments for a variety of inorganic particulate fillers, such as silicas, talcs, clays, and aluminas. Typical fillers include wollastonite, alumina tri-hydrate (ATH), magnesium hydroxide, kaolin, bentonite, precipitated or fumed silica, titanium dioxide, glass beads, and basalt. Silanes are also used to a lesser degree with fillers such as calcium sulfate, calcium carbonate, barium sulfate, graphite, and carbon black. Natural fibrous fillers, such as hemp, flax, or kenaf, may also be used with silanes.
Some of the more commonly used silanes for filler treatment include aminopropyltriethoxysilane (Silquest® A-1100), glycidoxypropyltrimethoxysilane (Silquest® A-187), ureidopropyltrimethoxysilane (Silquest® A-1524), beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (Silquest® A-186), methacryloxypropyltrimethoxysilane (Silquest® A-174), vinyl-tris-(2-methoxyethoxy)silane (Silquest® A-172) and vinyltriethoxysilane (Silquest® A-151), available from General Electric Company.
The use of alkoxy-functional silanes for the treatment of inorganic fillers results in the release of alcohol upon the hydrolysis of the silane, or reaction of the silane with the filler. This alcohol is typically methanol or ethanol, and can pose environmental, health, and safety concerns. As the emission of volatile organic compounds becomes more tightly regulated, filler treaters and compounders are many times forced to reduce production, install recovery or remediation equipment, or use special engineering controls to meet new, stricter emission limits as well as mitigate explosion or flammability hazards.
In some instances, conventional alkoxy-functional silanes can be prepared as stable, alcohol-free aqueous solutions, and these silane solutions can sometimes be utilized as filler treatments to reduce volatile emissions. An example would be the aqueous solution of aminopropyltrisilanol (Silquest® A-1106). However, in general, there are only a few examples where silanes can be prepared as stable, alcohol-free aqueous solutions. In addition, aqueous solutions of silanes typically result in increased costs to the end-user due to the extra processing steps required in their production, and the increased costs of shipment of the associated water.
Alternatively, silane oligomers may be used in some instances to reduce emissions of volatile organic compounds (VOCs) during the treatment of fillers. Again, only limited types of silanes are available as oligomers, and they do not always offer the same level of performance as conventional silanes.
Filler treaters and compounders need a more cost-effective way to reduce the emission of VOCs from their processes that employ conventional silanes.